My theory is that the past can't even be changed because: Let's say that you want to change something in the past. You change it, ok. But then you never had a reason to time travel, therefore you [i]didn't [/i] time travel and didn't change anything. About how dangerous it is, no one knows what actually happens when you time travel. There may be obstacles of some sort in the dimension(s) between past, present and future. And if you can travel through time safely, you may make some mistakes that leave the world in an entirely different shape than it was before you did it. Like, say, adopting Hitler for whatever reason. Just think how different things would be. [spoiler]To sum it up, I think it's dangerous???[/spoiler]